My answer is re law in Scotland, may be minor variations for England but Iprinciples should be the same. There is no law specifying an age when kids can be left on their own. However, parents have legal responsibilties towards their children including to safeguard their welfare 7 development & another to exercise their parental rights in the best interests of their children (see Children (Scotland) Act 1995 or Children Act). So having thought about the childs age &stage & their needs (eg for supervision, safety, protection, guidance, etc) the parent has to decide how best to meet those needs in any given circumstance. So think about each of your kids, the situation, what harm might it cause them,& what safeguards are their and make a decision on whether it is an ok situation. Ultimately they are your responsibility and if you leave them in a sitaution of actual or potential risk & the authorities becoem aware then you will be held accountable. Were you married to your ex and is he the childrens father? If so he will have the same rights & responsibilities as you but you can each exercise these independanatly of the other. D something about it!- talking to your ex would be the first step in this process. But if things are not amicable then you may need to get help from a family mediation service or a lawyer. The childrens needs are the priority. If you, your children and your ex have agreed (or court decided for you) that this contact is important for them then I think you all have to revisit the current arrangements in light of the fact that when he has the opportunity for contact with them your ex is off doing something else.You can leave a 16 year old to look after a youger child but again if anything hapens your choice of babysitter will be examined. You can also call your local child care social workers for some advice and guidance on the matter but i think you will get pretty much what I say above.